For devices operating in digital communications systems, achieving synchronization is important so that transmitted data may be accurately received. In many networks, information is transmitted in packets having embedded preambles. Preambles include sequences of values that are known to receiving devices. Upon receiving a packet, a device may process the packet's preamble to obtain timing synchronization. Further, the device may process the preamble to perform various operations, such as frequency offset estimation, preamble index detection, and/or channel estimation.
IEEE 802.16 WiMAX (Worldwide Interoperability for Microwave Access) networks employ preambles in their transmitted packets. Future WiMAX implementations (such as the IEEE 802.16m WiMAX II networks that are currently under development) will also employ preambles in their transmitted packets.
Current WiMAX systems interlace subcarriers to obtain three subcarrier sets. Each set utilizes every third subcarrier and provides a three sub-symbol periodicity in the time domain. Since Fast Fourier Transform (FFT) sizes are not divisible by three, this current approach requires a more complex receiver. Also, differential coding is not currently employed for WiMAX preambles, although some differential decoding algorithms have been proposed for the processing of received preambles.